The Mechanism Of Carbon Sequestration In Black Soil Under Integrated Management Of Straw And Nutrients

Returning crop straw to fields along with inorganic nutrients is a recommended measure to increase soil organic carbon(SOC)in agroecosystems.However,how to optimize nutrient management to drive a greater proportion of straw carbon into SOC and the underlying mechanisms are not fully known.Based on the typical farmland soil and the long-term positioning fertilization test platform in Northeast China,this study explored the characteristics of the ratio of carbon(C),nitrogen(N),phosphorus(P),and sulfur(S)at different points.Then,a nutrient supplementation experiment was conducted based on the stoichiometry ratio to clarify the intensity of newly formed soil-C(NFC,derived from straw)under different nutrient gradients and its underlying microbial metabolism mechanism.The major findings were as follows:1.There wss a relatively stable ratio of C:N:P:S in the fine fraction organic matter(FF-SOM)of typical black soil.Obvious differences were observed in the ratios of C:N,C:P,and C:S in the coarse fraction of SOM among Gongzhuling,Shuangcheng,Hailun,and Beian,and the coefficients of variation were 7.1%,22.2%,and 12.3%,respectively.The ratios of C:N,C:P,and C:S were relatively stable in FF-SOM at each site,and C had a strong linear regression relationship with N,P,and S(R²=0.96,0.76,and 0.90).In addition,the ratios of C to nutrient elements in the FF-SOM increased by6.8–15.4%after pickling with the aim to remove inorganic nutrients.Therefore,the accurate ratio of C:N:P:S could be obtained in the soil after pickling,and the ranges of C:N,C:P,and C:S ratio were12–13,58–65 and 66–79,respectively.2.Nutrient optimization management relieved the NP nutrient limitation of microbial metabolism and improved the microbial metabolism efficiency.Compared with the control soil,adding straw alone induced soil nutrient limitation,and the N and P acquisition extracellular enzyme activities(EEAs)increased by 3.1 times and 3.5 times,respectively.While C,N,and P EEAs increased almost synchronously under integrated management of straw and nutrients.Enzymatic stoichiometry revealed that microbial metabolism was mainly limited by N in the early stage(days 7,28),while the limiting effect of P gradually enhanced over time(especially Hailun).The N and P nutrient limitation of microbial assimilation of straw C was relatively alleviated after stoichiometric supplyof nutrient.Nutrient supplementation(from low to high)improved the microbial metabolism efficiency,supported by a decrease of-8.3%(-1.9–-17.8%)in qCO₂and an increase of 12.6%(1.5–23.6%)in C use efficiency.3.Nutrient supplementation improved the intensity of NFC,which operates depending on Acidobacteria and Proteobacteria and their investment in EEAs.The NFC increased from 1155.9 to1722.4 mg kg^-1 soil in Gongzhuling and increased from 725.1 to 1067.5 mg kg^-1 soil in Hailun as the levels of nutrient supplementation increased.Boosting regression tree analysis suggested that?-glucosidase(BG),acid phosphatase(AP),microbial biomass C(MBC),and Acidobacteria accounted for 27.8%,18.5%,14.7%,and 8.1%of the NFC in Gongzhuling and accounted for 25.9%,29.5%,10.1%,and 13.9%of the NFC in Hailun.Path analysis identified that Acidobacteria positively influenced NFC both directly and indirectly through regulating BG,AP,and MBC,in which MBC acquisition was more regulated by AP.4.Soil properties(SOC and pH)were also key factors affecting the microbial metabolism and NFC.The EEAs of C acquisition(BG)in Gongzhuling soil with lower SOC value were 1.2-1.8 times that of Hailun soil with higher SOC value.While the EEAs of P acquisition(BG)in Hailun soil with higher SOC value were 1.2-1.4 times that of Gongzhuling soil with lower SOC value,and the P acquisition enzyme showed stronger activities in different sampling periods in Hailun.The NFC intensity of Gongzhuling soil were 1.4-1.6 times that of Hailun soil with straw or nutrient addition.In addition,the NFC of Hailun soil was directly and negatively correlated with the changes in AP activity,which might be related to the effect of the lower pH and heavy loam of Hailun soil on the microbial activity and P availability.These phenomena indicated the importance of soil status(e.g.,SOC and pH)in determing the microbial metabolism strategy and NFC.In summary,the stoichiometric supply of NPS could promote the conversion of straw-C into NFC by alleviating the nutrient(N and P)limitation in microbial assimilation of straw-C,inducing the growth of major microbial taxa,increasing the activities of extracellular enzymes and improving the efficiency of microbial metabolism.This provides a scientific basis for the efficient use of straw resources and the optimal management of nutrients in the process of improving soil fertility in the black soil region.